CRON(8) BSD System Manager's Manual CRON(8)NAMEcron — daemon to execute scheduled commands (Vixie Cron)
SYNOPSIScron [-j jitter] [-J rootjitter] [-s] [-o] [-x debugflag[,...]]
The cron utility should be started from /etc/rc or /etc/rc.local. It
will return immediately, so you don't need to start it with '&'.
The cron utility searches /var/cron/tabs for crontab files which are
named after accounts in /etc/passwd; crontabs found are loaded into mem‐
ory. The cron utility also searches for /etc/crontab which is in a dif‐
ferent format (see crontab(5)).
The cron utility then wakes up every minute, examining all stored
crontabs, checking each command to see if it should be run in the current
minute. Before running a command from a per-account crontab file, cron
checks the status of the account with pam(3) and skips the command if the
account is unavailable, e.g., locked out or expired. Commands from
/etc/crontab bypass this check. When executing commands, any output is
mailed to the owner of the crontab (or to the user named in the MAILTO
environment variable in the crontab, if such exists).
Additionally, cron checks each minute to see if its spool directory's
modification time (or the modification time on /etc/crontab) has changed,
and if it has, cron will then examine the modification time on all
crontabs and reload those which have changed. Thus cron need not be
restarted whenever a crontab file is modified. Note that the crontab(1)
command updates the modification time of the spool directory whenever it
changes a crontab.
Enable time jitter. Prior to executing commands, cron will sleep
a random number of seconds in the range from 0 to jitter. This
won't affect superuser jobs (see -J). A value for jitter must be
between 0 and 60 inclusive. Default is 0, which effectively dis‐
ables time jitter.
This option can help to smooth down system load spikes during
moments when a lot of jobs are likely to start at once, e.g., at
the beginning of the first minute of each hour.
Enable time jitter for superuser jobs. The same as -j except
that it will affect jobs run by the superuser only.
-s Enable special handling of situations when the GMT offset of the
local timezone changes, such as the switches between the standard
time and daylight saving time.
The jobs run during the GMT offset changes time as intuitively
expected. If a job falls into a time interval that disappears
(for example, during the switch from standard time) to daylight
saving time or is duplicated (for example, during the reverse
switch), then it's handled in one of two ways:
The first case is for the jobs that run every at hour of a time
interval overlapping with the disappearing or duplicated inter‐
val. In other words, if the job had run within one hour before
the GMT offset change (and cron was not restarted nor the
crontab(5) changed after that) or would run after the change at
the next hour. They work as always, skip the skipped time or run
in the added time as usual.
The second case is for the jobs that run less frequently. They
are executed exactly once, they are not skipped nor executed
twice (unless cron is restarted or the user's crontab(5) is
changed during such a time interval). If an interval disappears
due to the GMT offset change, such jobs are executed at the same
absolute point of time as they would be in the old time zone.
For example, if exactly one hour disappears, this point would be
during the next hour at the first minute that is specified for
them in crontab(5).
-o Disable the special handling of situations when the GMT offset of
the local timezone changes, to be compatible with the old
(default) behavior. If both options -o and -s are specified, the
option specified last wins.
Enable writing of debugging information to standard output. One
or more of the following comma separated debugflag identifiers
must be specified:
bit currently not used
ext make the other debug flags more verbose
load be verbose when loading crontab files
misc be verbose about miscellaneous one-off events
pars be verbose about parsing individual crontab lines
proc be verbose about the state of the process, including all of
sch be verbose when iterating through the scheduling algorithms
test trace through the execution, but do not perform any actions
/etc/crontab System crontab file
/etc/pam.d/cron pam.conf(5) configuration file for cron
/var/cron/tabs Directory for personal crontab files
SEE ALSOcrontab(1), pam(3), crontab(5), pam.conf(5)AUTHORS
Paul Vixie ⟨email@example.com⟩
BSD June 17, 2007 BSD